1. Field of the Invention
The present invention relates to injection valves, and more particularly to a fuel-injection valve having a piezoelectric drive.
2. Description of the Prior Art
Valves having short reaction times are required in certain control executions. Injection valves having extremely short opening and closing times are particularly required for electronically-controlled fuel injection in internal-combustion engines in order to achieve short injection times (below 0.2 ms). Valves that only close slowly tend to form droplets and only offer low dosing accuracy.
Injection valves constructed in accordance with the art heretofore known generally comprise a valve drive based on the electromagnetic principle. Such a valve is opened by an electromagnet. A restoring spring closes the valve after the excitation is terminated. In order to achieve a short opening time, such valves are briefly driven with a high pulse having a high excitation current before a switch is undertaken to a low maintenance current. Due to the quadratic current/force behavior, the closing event, given an electromagnetic valve having a single magnetic coil, cannot be electrically influenced. It is solely dependent on the spring constant of the required restoring spring and on the mass of the valve needle to be moved.
The present invention is based on the perception that, in order to achieve short and defined opening and closing times, the valve can be actively opened and closed by employing a piezoelectric drive having an approximately linear relationship between the drive voltage for a piezoelectric actuator and the effected excursion. The desired proportionality of the injected quantity to the injection time can thereby be achieved, even given extremely-short injection times.
A disadvantage of piezoelectric piston generators, however, is that only relatively small excursions (0.1%-0.2%) can be achieved, so that they cannot be directly employed as actuators for valves.